Recombinant approaches have been used in attempts to develop vaccines against diseases for which no vaccine currently exists, or for which conventional vaccine approaches are less desirable. For example, since the human immunodeficiency virus (HIV) was first identified as the etiologic agent of Acquired Immuno-deficiency Disease Syndrome (AIDS), (Barre-Sinoussi et al. Science 220:868 (1983); Levey et al., Science 225:840 (1984); Gallo et al., Science 224:500 (1984)), considerable effort has been directed towards the development of a safe and effective vaccine.
The human immunodeficiency viruses, HIV-1 and HIV-2, are members of the lentivirus subclass of retroviruses. Gonda et al., Science 227:173 (1985); Sonigo et al., Cell 42:369 (1985). The virus particles contain an inner core comprised of capsid proteins (encoded by the viral gag gene) that encase the viral RNA genome. Rabson & Martin, Cell 40:477 (1985). The central core is surrounded by a lipid envelope that contains the viral-encoded envelope glycoproteins. Virus-encoded enzymes required for replication, such as the reverse transcriptase and integrase (encoded by the pol gene), are also incorporated into the virus particle.
There are obvious difficulties with the use of whole virus for an HIV vaccine. The fear that an attenuated virus could revert to virulence, and the danger of incomplete inactivation of killed virus preparations, together with the reluctance to introduce the HIV genome into seronegative individuals have argued against the uses of live attenuated or killed HIV vaccines for the prevention of infection.
Advances in recombinant DNA technology may make it possible to use heterologous expression systems for the synthesis not only of individual antigens, but also of defective, nonself-propagating, virus-like particles. It has been demonstrated that capsid proteins of certain viruses can assemble into particles morphologically and immunologically similar to the corresponding virus. For example, the P1 precursor of several picornaviruses synthesized in vitro can be processed into individual capsid proteins which then assemble into immunoreactive virion-like particles. Nicklin al., Biotechnology 4:33 (1986); Palmenberg et al., J. Virol. 32:770 (1979); Shih et al., Proc. Natl. Acad. Sci. USA 75:5807 (1978); Hanecak et al., Proc. Natl. Acad. Sci. USA 79:3973 (1982); Grubman et al., J. Virol. 56:120 (1985). Self-assembly of capsid proteins expressed in vivo in several recombinant expression systems has also been reported. For example, when human hepatitis B surface antigen is expressed in yeast cells, the polypeptide assembles into particles similar in appearance to those isolated from human plasma (Valenzuela et al., Nature 298:347 (1982)); these particles stimulate anti-hepatitis B antibody production in several species and can protect chimpanzees from virus challenge. McAleer et al., Nature 307:178 (1984).
In another example, it was shown that coexpression of canine parvovirus (CPV) capsid proteins VP1 and VP2 in murine cells transformed with a bovine papilloma virus/CPV recombinant plasmid resulted in the formation of self-assembling virus-like particles (Mazzara et al., 1986, in Modern Amproaches to Vaccines, Cold Spring Harbor Laboratory, N.Y.; R. M. Chanock and R. A. Lerner, eds. pp. 419-424; Mazzara et al., U.S. patent application Ser. No. 905,299, filed Sep. 8, 1986); when used to vaccinate susceptible dogs, these empty capsids elicited immune responses capable of protecting against CPV challenge. It has also been shown that the HIV-1 and SIV p55gag precursor polypeptides expressed in Spodoptera frugiperda cells using a baculovirus expression vector assembles into virus-like particles which are secreted into the cell culture medium. Gheysen et al., Cell 59:103 (1989); Delchambre et al., The EMBO J. 8:2653-2660 (1989).